Stump cutting machines are well known for removing undesirable tree stumps. Such machines include a rotating cutter wheel driven by a gas or diesel engine. The cutter wheel, while rotating, is advanced toward the stump and moved laterally across the face of the stump. The cutter wheel is mounted at one end of a boom which is, in turn, pivotally mounted on a support frame. Hydraulic boom swing cylinders are used to pivot the boom about the pivot point to move the cutter wheel back and forth across the face of the stump to cut it away. The hydraulic boom swing cylinders are controlled by a control lever handle. Other handles control advancing the cutter wheel into the stump (e.g., in and out) and up/down. Further, the handles can be used to provide an additional function, namely determining operator presence.
Operator Presence Systems (OPS) are also known in the art. Typically, the systems include two main components: a sensor system activated by an operator when physically located at an operating station, and deactivated when the operator leaves the operating station; and a control system that disables a powered element of the machine when the sensor system is deactivated.
In the case of a stump cutter with hydraulic cutter wheel positioning controls, the effectiveness of an OPS is influenced by the configuration of the controls and the configuration of the operator station. More specifically, operation of the stump cutter requires activation of three machine movements that can be described as corresponding to an x, y, z axis system, including:                x-direction: sweep control can generally be described as a first direction generally parallel to the ground. The cutter wheel is typically moved by pivoting a support arm about an axis of rotation. Although this movement is generally not linear, it can be generally described as movement in a first direction;        y-direction: movement of the complete machine, forwards or backwards, can be described as a second direction, also generally parallel to the ground, and described as movement in the y-direction; and        z-direction: raising or lowering the cutter wheel can generally be described as the third direction (e.g., the z-direction).These three movements are provided by 3 separate control levers which the operator controls using both hands. The swing control is used almost constantly with one of the operator's hands while the other functions are subject to frequent, intermittent and sometimes simultaneous use with the other hand for proficient operation.        
Operator stations and control requirements on other types of machines that commonly utilize an OPS typically do not pose such unique challenges. For instance, ride-on machines typically use a sensor which simply relies upon body weight, or pedestrian machines such as push lawnmowers which simply require squeezing a bail or handle(s) against the handlebar with one or two hands. Neither example significantly complicates the operator's efforts to control the machine.
The operator station of a stump cutter is determined by the location of the three hydraulic control levers described above—with the operator standing on the ground. In this position, the operator is located in an area where the risk of being struck by a thrown object is minimized. During operation, the entire machine is advanced into the stump thus requiring the operator to alter his/her standing position to remain comfortably in front of the controls. Job site conditions can also affect how the operator is positioned at the control area. In addition, to avoid fatigue while standing for potentially long periods of time, it is important that the operator be able to comfortably reposition his/her feet and stance.
Sensing the operator's presence at the control levers of a stump cutter is further complicated by the intermittent, and sometimes simultaneous, use of the three control levers. Additionally, operators routinely use gloves for protection from the weather or as protection for an operator's hands. The combined effect results in a situation where the option of utilizing levers with mechanical or electromechanical interlocks is not practical. These devices would require an operator to depress a button or switch while gripping the lever each time the operator moved from one control lever to another. Such a system also provides little freedom in repositioning the operator's hands on the levers. Accordingly, such a system would cause fatigue, result in frequent unwanted cutter wheel stoppages, hinder operation, and create control errors. Freedom of hand/finger position on the control levers is also very important to avoid hand fatigue. Such difficulties are likely to cause operator rejection and provide motivation for circumventing the system.
Therefore, there is a need in the art for an OPS for a stump cutter device that will not inhibit the operator's control of the stump cutter, will provide reliable operation, and can appropriately sense the operator's presence at the control levers without creating fatigue and operator discomfort. The present invention overcomes the shortcomings of the prior art, and addresses these needs.